The present invention relates generally to heat pump equipment for heating and cooling applications. More particularly, the invention relates to an electronic control system for optimizing compressor motor performance and for handling sensor faults while maintaining a minimum negative impact on system performance and reliability.
Conventionally air-conditioning and heat pump systems must be designed to handle extreme conditions which only infrequently occur. There is, for example, the maximum load/low voltage condition which occurs only on very hot days. On very hot days, the compressor discharge pressure is high and the suction pressure on the load side (indoors) is also high. High compressor motor torque is therefore required to meet these extreme pressure demands, which in turn dictates that the system be designed with an oversized compressor motor. Exacerbating this condition is the frequent low voltage or brownout condition that occurs in many parts of the country during hot days. The low voltage or brownout condition places an additional strain on the motor.
As a result of these maximum load/low voltage demands, system manufacturers have traditionally designed their systems with larger motors than would otherwise be required, if the extreme conditions could be guaranteed never to occur. The need to use these larger motors increases the cost of the system. Moreover, the use of larger motors actually decreases the overall system efficiency, since a motor designed to handle the high torque, low voltage extremes typically does not operate at peak efficiency under the less extreme conditions normally encountered. In effect, the need to accommodate the maximum load and low voltage conditions constrains the system designer to use a less efficient motor.
With the continuing effort to improve system efficiency, under extreme conditions and normal conditions alike, there is a need for ways to optimize motor efficiency. Conventional heating and cooling systems have been generally deficient in this regard.
Another area where heating and cooling systems could be improved is in the detection and handling of sensor faults. Today's HVAC equipment is becoming comparatively complex. Most systems use one or more sensors or transducers in conjunction with a control system which is intended to keep the system operating efficiently while meeting the heating and cooling demands of the load. Sensor malfunctions are therefore a significant problem.
In a conventional heat pump system, for example, a sensor may be used to implement the defrost function of the outdoor coil in heating mode. If this sensor malfunctions, the heat pump system can begin to improperly perform the defrost function. This can lead to blockage of the outdoor coil from frost buildup resulting in significant loss in heating performance. Such a conventional system does not have the capability to take corrective action or to alert the user that a sensor malfunction has occurred. Thus, a sensor malfunction may not become apparent until there is visual inspection of the outdoor unit of the heat pump system, or until the user notices higher electric bills from the lost heating performance, which must be made up by electric resistance heaters.
The present invention addresses this and other sensor fault problems by providing a microprocessor-controlled system operating mechanism which monitors the integrity of system sensors and keeps the heat pump system operational in alternate modes each designed to have a minimum negative impact on system performance and reliability. The system operating mechanism automatically selects the mode of operation, based on which sensor or sensors have been found to be malfunctioning. In addition, the system operating mechanism also provides an early warning to the user by displaying a sensor malfunction code or codes on the room thermostat. In addition to alerting the home owner that an error has occurred, the malfunction code is a time saving diagnostic tool for the technician during servicing of the system.
Accordingly, in one aspect the invention provides a system operating method in which the discharge temperature of the refrigerant discharged from the compressor is obtained and compared with a predetermined temperature indicative of an alert condition. Based on the comparing step, if the discharge temperature is above the predetermined temperature, the discharge temperature is used to control the setting of the heat pump system expansion valve. On the other hand, if the discharge temperature is not above the predetermined temperature (0.degree. F.), the electronic expansion valve (EXV) is set to a predetermined setting.
According to another aspect of the invention, a sensor is used to obtain an outdoor air temperature and this outdoor air temperature is compared with a predetermined temperature indicative of a sensor fault condition. Based on the comparing step, if the outdoor air temperature is above the predetermined temperature, the outdoor air temperature is used to control the speed of the indoor fan or blower. On the other hand, if the outdoor air temperature is not above the predetermined temperature (-77.degree. F.), the fan is run at a predetermined speed and the EXV set to a predetermined opening.
In yet another aspect of the invention, a sensor is used to obtain an outdoor coil temperature and this temperature is compared with a predetermined temperature indicative of an alert condition. Based on the comparing step, if the outdoor coil temperature is above the predetermined temperature, the outdoor coil temperature is used to control operation of the coil defrosting system. On the other hand, if the outdoor coil temperature is not above the predetermined temperature (-77.degree. F.), the defrosting system is periodically operated at predetermined time intervals.
The aforementioned sensor fault handling methods may be implemented separately or in various combinations depending upon the complexity of the heat pump system. As more fully set forth below, the sensor fault handling methods can be applied in both heating mode and cooling mode.
In yet another aspect of the invention, a system operating mechanism is provided which checks for the existence of conditions indicative of a maximum load/low voltage condition and which automatically opens the expansion valve to increase refrigerant flow. This has the beneficial effect of cooling the system. By thus providing automatic system cooling it is possible to implement an HVAC system more economically since the heat pump motor can be sized and optimized for normal conditions instead of abnormal conditions.